Silicon-germanium heterobipolar transistors in which the base electrodes are formed by a silicon-germanium layer feature substantially higher cut-off frequencies than transistors fabricated by conventional methods, since the injection of germanium atoms into the base layers of the transistors reduces the band gap energy. The silicon-germanium base layer is normally applied either by implantation or, like the emitter layer thereabove, by epitaxial growth. Provided between the silicon-germanium base layer and the emitter layer is a silicon dioxide layer. The oxide of the silicon dioxide layer is normally produced by thermal oxidation of silicon in tube furnaces in which batches of up to 200 wafers are oxidized simultaneously. The thermal inertia of these furnaces necessitates lengthy processing, resulting in the wafers being maintained at very high temperatures for a relatively long time. The electronic properties of bipolar transistors are determined, among other things, by the properties of this silicon dioxide layer.